Mapping Migration in the United States

On Thursday, we published a series of interactive charts showing how Americans have moved between states since 1900. The charts show striking patterns for many states: You can trace the rise of migrant and immigrant populations all along the Southwest, particularly in Texas and Arizona; the influx of New Yorkers and other Northeasterners into Florida starting in the 1970s; and the growth in the Southern share of the Illinois population during the Great Migration.

In 1900, 95 percent of the people living in the Carolinas were born there, with similarly high numbers all through the Southeast. More than a hundred years later, those percentages are nearly cut in half.

Taken individually, each state tells its own story, and each makes for fascinating reading. As a follow-up, here is the big picture: a map showing all of the states at a given time.

Each shape represents where the people living in a state were born. Within a state, larger shapes mean a group makes up a larger share of the population.

For map enthusiasts out there — you know who you are — this is a new kind of chart we are calling a Voronoi treemap map, based on the work of two visualization researchers, Michael Balzer and Oliver Deussen.

The Times has published treemaps before, as when it described how financial companies lost value during the financial crisis; it has also published Voronoi treemaps before, notably to describe inflation in 2008. But this is the first Voronoi treemap map, which uses geographic shapes as the boundaries in a Voronoi treemap.

One drawback of this technique, however, is that it’s not always possible to make each shape exactly the right size; some small values are slightly larger or smaller than they should be. (It’s either that or put holes in the map.)